Human-body communication is a technique for transmitting signals between apparatus connected to a human body by using the human body having conductivity as a communication channel. In the human-body communication technique, a communication network to various portable apparatuses such as personal digital assistants (PDAs), portable personal computers, digital cameras, MP3 players, and mobile phones or a communication network to fixed-type apparatuses such as printers, TVs, and entrance systems can be implemented by a user simply contacting the apparatuses.
In a conventional human-body communication technique, low-rate data communication having a maximum transmission rate of tens Kbps can be implemented by using a digital modulation scheme such a frequency shift keying (FSK) modulation scheme or an amplitude shift keying (ASK) modulation scheme and a specific frequency. In addition, applications thereof are limited to simple applications requiring only the data transmission.
A human body is constructed with various materials and shapes and has a low conductivity and a high dielectric constant. Therefore, the human body has a function as an antenna in a wide frequency band. Due to the characteristics of the human body, signals of other users near to the human body or noise signals originated from external electronic apparatuses may be induced into the human body. In addition, the communication may be unstable according to positions of neighboring objects and electronic apparatuses and distances among them.
In the frequency modulating scheme such as FSK, ASK, and PSK modulation schemes, a frequency band having less external interference can be selected and used, and a relatively low signal-to-noise ratio is required for implementation of communication. However, if interference occurs due to the characteristics of human body, it is difficult to avoid the interference.
In addition, in a direct transmission scheme of digital signals such as the human-body communication scheme, a very wide frequency band is needed, and a receiving signal is mixed with strong interference induced from neighboring electronic apparatus as well as the transmitting signal transmitted from a transmitter. In case of occurrence of the interference in the signal band, even a receiver having a good sensitivity cannot easily extract desired signals.
In addition, in the direct signal transmission scheme, a frequency band of several tens MHz or more is needed to directly transmit the digital signals of several Mbps or more, and when such signals are applied to the human body, signals in the band to exceed a specific frequency are radiated outward body. Therefore, in a case where a plurality of users is located, even though there is no contact, signals of other users occur interferences, so that it is difficult to implement a stable communication network.